Carbon dioxide monitoring of patients respired gases, during anesthesia and the display of its numerical value is referred to as Capnometry and Capnography, the system which adds a graphic display of the instantaneous concentration of carbon dioxide, is now generally accomplished by using non-dispersive Infra-red or IR Spectrometry techniques. Two broad classes of measurement systems have been developed:
(a) The first, side-stream monitors, is the type where the measurement is performed in the IR monitor upon a sample of gas.
This sample of gas is continuously drawn from the patients airway by means of a suction pump. The suction pump is connected by way of a small diameter tube to a T-piece, placed on the tracheal tube; anesthesia mask connector; or through a tube introduced into the pharynx through the patient's nostril. Compensation or scavenging must be provided in the carbon dioxide monitor to prevent spurious readings due to the effects of atmospheric pressure; halogenated anesthetic vapors; nitrous oxide; and/or water vapor. However another and a greater source of error or malfunction is due to the clogging of the tubing necessary to convey the sample gas from the patient to the monitoring instrument. In practice, it is desirable that this tubing be a small diameter capillary tube to minimize "dead volume". The "dead volume" is the volume of gas in the tubing between the sampling tube and monitor or measuring device. This "dead volume" must be completely displaced to update the instantaneous measurements. During the displacement of the "dead volume", this tubing becomes susceptible to clogging and/or occlusion due to condensed water vapors and/or patient secretions.
In some monitors or devices, an alarm is sounded when clogging becomes excessive. Compensating corrections in some instruments may take the form of either reversing air-flow to clear moisture or secretions out of the tubing, or increasing of the sample flow to act as a purge. However, since side-stream instruments have ample room within them, scavenging, liquid traps, calibration and compensation means can be readily incorporated at the cost of slower response time, while not reducing the risk of clogging in the long sampling tube. In addition, the patients breath and secretions drawn into the instrument may come into contact with the permanent pneumatic components and thus pose a possibility of cross-contamination between patients which then must be dealt with.
(b) The second class of CO.sub.2 monitor, the main stream type, incorporates a special breathing circuit insert (cuvette) for attaching the IR sensor to the Y-piece of the breathing circuit.
This main stream type directs the IR from an IR source or sources, of an appropriate wavelength, to pass directly through the airway where its intensity is attenuated by the gas concentration and measured at the opposite end of the breathing tube. While this second class of CO.sub.2 monitor provides a faster and more direct reading of the gas concentration then the side-stream type instruments, it also presents another problem. The problem of the main-stream type instrument is the need to incorporate an IR source and detector in addition to the compensation and calibration circuitry into the device itself. This device which is unsupported, may significantly increase the traction placed on the patients airway by the tracheal tube and therefore making for a complicated and expensive sensing device.